Field of the Invention
The invention relates to a gas laser, in particular an axial-flow CO.sub.2 gas laser, having a resonator with a beam path folded in several planes.
It is known from Published UK Patent Application 2 090 047 A that it is expedient to use a circularly polarized laser beam for material processing employing a laser beam, in particular for cutting a workpiece, since in that case the cutting quality when cutting by using a laser is independent of the cutting direction. In the case of the laser known from that Published UK Patent Application, such a circular polarization is generated by using a phase-shifting mirror inclined at 45.degree. to the direction of propagation to convert a linearly polarized laser beam emerging from the resonator of the laser into a circularly polarized laser beam. For that purpose, the direction of polarization of the linearly polarized laser beam must be at an angle of 45.degree. to the plane of incidence defined by the normal of the phase-shifting mirror and the direction of incidence on the phase-shifting mirror. The known CO.sub.2 gas laser has a folded beam path which has a Z-fold in a folding plane. A folding mirror of the Z-fold is disposed with its normal at 45.degree. to the plane defined by the Z-fold, with the result that the laser beam incident on it from a Z-limb defines in common with the normal a plane which is orientated at 45.degree. to the plane of the Z-fold. In that way, the beam emerging from the resonator is already polarized at 45.degree. to the plane of the Z-fold. The laser beam emerging from the resonator is converted through the use of the phase-shifting mirror into a circularly polarized beam which is then directed onto a workpiece through further deflecting mirrors external to the resonator.
It is known from European Patent 0 121 661 B1, corresponding to U.S. Pat. No. 4,634,831, to place a flat resonator with a beam path folded in a multiple U-shaped manner at 45.degree. to the horizontal. Therefore, upon leaving the resonator, the direction of polarization of the laser beam emerging from the resonator and linearly polarized, as a consequence of the U-fold, perpendicular to the plane defined by the U-fold, is already at 45.degree. to the horizontal. That laser beam which is linearly polarized at 45.degree. can then both be deflected by 90.degree. towards a generally horizontally disposed processing surface of a workpiece, and also converted into a circularly polarized laser beam, through the use of a phase-shifting mirror. Therefore, such a configuration eliminates the polarization mirror which is required at the end of a limb on the Z-fold in the resonator according to Published UK Patent Application 2 090 047 A.
Published European Patent Application 0 591 541 A1 has disclosed a CO.sub.2 gas laser with a resonator having a beam path folded in two mutually perpendicular U-planes. The two U-planes are disposed at 45.degree. to the horizontal and folded with one another. The two U-folds are connected to one another through a coupling mirror, the normal of which is vertically orientated. In that way, a laser beam is produced which is linearly polarized at 45.degree. to the vertical. German Published, Non-Prosecuted Patent Application DE 37 22 256 A1 has disclosed a laser resonator which has two U-folds that are disposed parallel to one another in mutually separated planes and are connected to one another through a coupling section extending diagonally at 45.degree. to the U-folding planes. The diagonal coupling section connects limbs of the two U-folds which are opposite one another diagonally. It effects a rotation of the beam by 90.degree., with the result that the s and p-components of the upper U-folding plane are transformed into respective p and s-components of the lower U-folding plane. That results in mutual compensation of the polarization effects of the 90.degree. reflecting mirrors disposed in the U-folding planes, thereby leaving a 45.degree. polarization produced by the diagonal reflection. In addition to the linear polarization of the output beam at 45.degree. to the horizontal, other requirements which must be fulfilled by a CO.sub.2 gas laser that is to be used, in particular for cutting applications, are a rotationally symmetrical radiance distribution as well as a high beam quality. A high beam quality requires a low Fresnel number N.sub.F =a.sup.2 /(.lambda..multidot.L), wherein a is the aperture, .lambda. is the wavelength and L is the resonator length. In practice, the resonator length used for high-power CO.sub.2 gas lasers is about 5-10 m. The construction of a compact high-power CO.sub.2 gas laser is thus possible only with a resonator which has a multiply folded beam path. However, a multiply folded beam path such as is known from the printed publications mentioned at the outset, has the disadvantage of requiring a multiplicity of deflecting or folding mirrors in the resonator, which make the laser more expensive and lead to a reduction in efficiency due to absorption that takes place in them. A multiplicity of deflecting or folding mirrors also complicates the adjustment of the resonator. Moreover, there is a need for a complicated mechanical structure which absorbs the thermal stresses produced by heating during operation, without that entailing maladjustment of the sensitive optical components in the resonator.